The present invention relates to an electronic circuit device for processing a small electric signal, and more particularly to an electronic circuit device having a shielding unit for protection of the circuit from electromagnetic wave noise, and a noise removing apparatus.
Such an electronic circuit device is used in an apparatus for measuring the flow rate of inflow air, an apparatus for measuring a negative pressure of an inlet pipe, an apparatus for measuring acceleration, or the like, for example a device of the type used with an internal combustion engine.
There is described in JP-A-58-6414, JP-A-61-9012, and JP-A-1-97817 an electronic circuit device which is used in an apparatus for measuring the flow rate of inflow air provided in an internal combustion engine.
The electronic circuit device described in the above-referenced patent publications is provided with a shielding casing which is made of metal for surrounding an electronic circuit portion of the device in order to protect the electronic circuit from electromagnetic wave noise, and a feedthrough ceramic capacitor which is used for forming a filter between an I/O signal line and a terminal of a module of the electronic circuit apparatus.
In the prior art electronic circuit device, the provision of the shielding casing prevents the module from being miniaturized or made more compact.
Moreover, the feedthrough ceramic capacitor is fixedly mounted to a supporting plate (some of them also act as a shielding casing) which is perpendicularly mounted to a base of the electronic circuit. Then, since this supporting plate is present even if the shielding casing can be omitted, there is a limit to possible miniaturization of the device.
Further, there arises a problem in which since the fact that, after a high frequency current induced in a wire harness by a strong electric field of high frequency has passed through the feedthrough ceramic capacitor, the current is not attenuated sufficiently in magnitude, with the result that the electronic circuit in the shielding casing malfunctions.
Moreover, there is encountered a problem in that it is difficult to automatically mount the feedthrough ceramic capacitor, not to mention the fact that the mounting of the feedthrough ceramic capacitor hinders the other elements from being automatically assembled.
On the other hand, the output of such an electronic circuit apparatus is fed to a signal processing apparatus which is separately provided. Then, in the prior art apparatus, the form of the output signal is not directly adapted to the signal processing apparatus. Therefore, it may be necessary in some cases to provide a special interface in the I/O port of the signal processing apparatus, and to perform a preprocessing in the processing circuit. In order to solve this problem, the output of the electronic circuit apparatus is preferably outputted through an open-collector type transistor. However, in this case, there arises a problem in that the total impedance of the electronic circuit viewed from the output side is readily influenced by electromagnetic wave noise, and therefore, with the prior art filter, even if a shielding casing is provided, the presence of electromagnetic wave noise superimposed on the output signal line can not be sufficiently provided.